Technology update

Feb 7, 2008

Stacked nanotubes enhance field emission

Scientists in the US have enhanced carbon nanotube (CNT) field emitters by growing single-walled and thin multi-walled structures on top of an array of multi-walled material. The CNT-CNT multistage design offers a field emission current that is 32 times the value of a similar structure based solely on multi-walled CNTs as well as providing a lower turn-on field.

"These parameters are important as they allow designers to reduce the operating voltage and cost of the device", Wonbong Choi, director of Florida International University's nanomaterials and device laboratory told nanotechweb.org. "By applying one-third of the voltage, we can get the same level of electric field thanks to the concentrating effect of our multistage single-walled/multi-walled CNT emitter."

Choi expects that multistage nanotube arrays will eventually replace thermionic cathodes currently found in high-power microwave devices, dramatically reducing the size and weight of long-distance communications hardware. Other applications include flat panel displays and electron guns for next-generation microscopes.

The researchers grow their CNT multistage emitters on porous silicon by catalytic thermal chemical vapour deposition. To form the multi-walled CNT array, iron catalyst is sputtered through a shadow mask on to the silicon substrate. The treated sample is then annealed, stabilized and exposed to C2H2. Single-walled tubes are encouraged to grow on top of the now present multi-walled structures by re-aligning the shadow mask over the array and once again depositing iron catalyst. This time the sample is exposed to a mixture of CH4 and C2H4.

TEM and Raman analysis verified the existence of single-walled CNTs and thin multi-walled CNTs on top of the emitter array.

To further improve the emission current, Choi and his team plan to synthesize the CNTs on n-type porous silicon. The material is harder to work with than p-type silicon (as used here), but should increase the efficiency of electron injection from the substrate across the nanotube interface.